Grapevine leafroll disease is found everywhere grapevines are grown and, in economic terms, is one of the most important diseases in the grape industry. Grapevine leafroll disease is associated with reduced yield, delayed ripening, altered fruit pigmentation, and reduced accumulation of sugar.
Several graft transmissible viruses have been reported to be associated with leafroll disease including a potyvirus (GPV), different strains of closterovirus-like particles (GLRaV-1, 2b, 3, 4, 5), grapevine virus A (GVA), and an uncharacterized isometric virus. Closterovirus-like particles, designated as grapevine corky bark associated viruses (GCBaV), have also been reported in association with corky bark disease. Grapevine cultivars often carry mixed infections of unrelated GLRaV (grape leafroll-associated viruses) and GCBaV.
Because of the destructive effects of leafroll disease, it is necessary to screen plants for the presence of leafroll disease associated-viruses, particularly before carrying out procedures such as bench-grafting, budding and varietal conversion which may spread the disease from one plant to another. Presently, diagnosis of infection by leafroll disease-associated viruses is done by biological indexing and ELISA tests. Biological indexing utilizes grape selections, called "indicator" varieties, which are especially sensitive to particular virus diseases. In an indexing test, buds from the grape selection being tested are grafted to potted vines of the indicator variety. The grafted plants are then grown in a field plot: if the tested selection has leafroll disease, the indicator vines will develop symptoms in approximately two years. Indexing is not a practical diagnostic tool for use on a commercial basis because of the length of time required for disease expression.
More rapid tests for leafroll disease (LD) associated viruses are immunological tests such as the Enzyme Linked Immunosorbent Assay (ELISA). However, because the known LD-associated viruses are distinct, ELISAs are specific for each virus and are, of course, available only for known viruses. Because antibodies are not available for all viruses a negative ELISA test may not be indicative of a virus-free plant. Other tests being developed, such as nucleic acid based tests, also are useful only for detecting known and characterized viruses.
Therefore, there is a need to identify new viruses associated with LD, and for improved assays for LD-associated viruses, especially assays directed to heretofore uncharacterized viruses. Molecular and immunological probes for detection of grapevine leafroll associated viruses will allow economical and reliable testing of grapevine nursery material and elimination of viruses from nursery stock.